An electrophotographic image forming apparatus primarily transfers a toner image formed on a photosensitive body onto an intermediate transfer body, then secondarily transfers the toner image onto a printing medium such as a paper sheet, and fixes the toner image on the printing medium, thus obtaining a toner image. The intermediate transfer body may have a drum or belt shape. Since the belt-shaped intermediate transfer body is advantageous in term of space, a size reduction of the image forming apparatus can be attained.
In order to obtain a full-color image by an image forming apparatus that uses an intermediate transfer belt, toner images of three colors, i.e., yellow, cyan, and magenta, or four colors also including black in addition to these three colors, are primarily transferred in turn from a photosensitive body into the intermediate transfer belt, and a full-color toner image overlaid on the intermediate transfer belt is secondarily transferred onto a printing medium at the same time.
In this case, if the overlaying positions of the toner images of the three or four colors deviate from each other, the tint or the like of the obtained image becomes different from the original image or the like. For this reason, in order to obtain high image quality of the full-color image, multi-color toner images to be overlaid on the intermediate transfer belt must be accurately aligned.
As an example of aligning the overlaying positions of the multi-color toner images on the intermediate transfer belt, a method of forming an image by measuring the perimeter of the intermediate transfer body is known. According to Japanese Patent Laid-Open No. 10-123846, in order to measure the belt perimeter of the intermediate transfer body, a mark is formed on the intermediate transfer body and is detected from the rotating intermediate transfer body in advance, and the belt perimeter is calculated based on the detection time interval (mark detection period) and the velocity of the intermediate transfer body.
However, in order to count the mark detection period, when a counter or the like is provided to execute sampling based on counter source clocks, that sampling period is reflected on the belt perimeter value as a quantization error (perimeter error). When color overlaying processes are executed on the basis of the belt perimeter value including the quantization error, quantization errors are accumulated for three rotations of the intermediate transfer body, i.e., those for the first color→the second color, the second color→the third color, and the third color→the fourth color. As a result, at the final transfer position of the fourth color, the quantization errors (=quantization error×3) of the belt perimeter value for three rotations influence color misregistration at the transfer position.